Revision control (also known as version control (system) (VCS), source control or (source) code management (SCM)) is the management of multiple revisions of the same unit of information. It is most commonly used in engineering and software development to manage ongoing development of digital documents like application source code, art resources such as blueprints or electronic models and other critical information that may be worked on by a team of people. Changes to these documents are identified by incrementing an associated number or letter code, termed the "revision number", "revision level", or simply "revision" and associated historically with the person making the change. A simple form of revision control, for example, has the initial issue of a drawing assigned the revision number "1". When the first change is made, the revision number is incremented to "2" and so on.

Standalone version control systems mostly come from the software engineering industry, but revision control is also embedded in various types of software like word processors (e.g. MSWord, OOwriter, Koffice), spreadsheets (e.g. OOcalc), in various content management systems. Integrated revision control is a key feature of wiki software packages such as MediaWiki, TWiki, etc. In wikis, revision control allows for the ability to revert a page to a previous revision, which is critical for defending a public wiki against vandalism and spam, to allow legitimate users to correct their mistakes, and to allow groups of editors to track each other's edits.

Software tools for revision control are increasingly recognized as being necessary for the organization of multi-developer projects.^[1]

Overview

Engineering revision control developed from formalized processes based on tracking revisions of early blueprints or bluelines. Implicit in this control was the ability to return to any earlier state of the design, for cases in which an engineering dead-end was reached in the development of the design. Likewise, in computer software engineering, revision control is any practice that tracks and provides control over changes to source code. Software developers sometimes use revision control software to maintain documentation and configuration files as well as source code. Also, version control is widespread in business and law. Indeed, "contract redline" and "legal blackline" are some of the earliest forms of revision control, and are still employed with varying degrees of sophistication. An entire industry has emerged to service the document revision control needs of business and other users, and some of the revision control technology employed in these circles is subtle, powerful, and innovative. The most sophisticated techniques are beginning to be used for the electronic tracking of changes to CAD files (see Product Data Management), supplanting the "manual" electronic implementation of traditional revision control.

As software is designed, developed and deployed, it is extremely common for multiple versions of the same software to be deployed in different sites, and for the software's developers to be working simultaneously on updates. Bugs and other issues with software are often only present in certain versions (because of the fixing of some problems and the introduction of others as the program develops). Therefore, for the purposes of locating and fixing bugs, it is vitally important to be able to retrieve and run different versions of the software to determine in which version(s) the problem occurs. It may also be necessary to develop two versions of the software concurrently (for instance, where one version has bugs fixed, but no new features, while the other version is where new features are worked on).

At the simplest level, developers could simply retain multiple copies of the different versions of the program, and number them appropriately. This simple approach has been used on many large software projects. While this method can work, it is inefficient as many near-identical copies of the program have to be maintained. This requires a lot of self-discipline on the part of developers, and often leads to mistakes. Consequently, systems to automate some or all of the revision control process have been developed.

Moreover, in software development and other environments, including in legal and business practice, it is increasingly common for a single document or snippet of code to be edited by a team, the members of which may be geographically diverse and/or may pursue different and even contrary interests. Sophisticated revision control that tracks and accounts for ownership of changes to documents and code may be extremely helpful or even necessary in such situations.

Another use for revision control is to track changes to configuration files, such as those typically stored in /etc or /usr/local/etc on Unix systems. This gives system administrators another way to easily track changes to configuration files and a way to roll back to earlier versions should the need arise.

Compression

Most revision control software can use delta compression, which retains only the differences between successive versions of files. This allows more efficient storage of many different versions of files.

Source management models

Traditional revision control systems use a centralized model, where all the revision control functions are performed on a shared server. If two developers try to change the same file at the same time, without some method of managing access the developers may end up overwriting each other's work. Centralized revision control systems solve this problem in one of 2 different "source management models": file locking and version merging.

File locking

The simplest method of preventing "concurrent access" problems is to lock files so that only one developer at a time has write access to the central "repository" copies of those files. Once one developer "checks out" a file, others can read that file, but no one else is allowed to change that file until that developer "checks in" the updated version (or cancels the checkout).

File locking has merits and drawbacks. It can provide some protection against difficult merge conflicts when a user is making radical changes to many sections of a large file (or group of files). But if the files are left exclusively locked for too long, other developers can be tempted to simply bypass the revision control software and change the files locally anyway. That can lead to more serious problems.

Version merging

Most version control systems, such as CVS, allow multiple developers to be editing the same file at the same time. The first developer to "check in" changes to the central repository always succeeds. The system provides facilities to merge changes into the central repository, so the improvements from the first developer are preserved when the other programmers check in.

The concept of a reserved edit can provide an optional means to explicitly lock a file for exclusive write access, even though a merging capability exists.

Distributed revision control

Distributed revision control takes a peer-to-peer approach, as opposed to the client-server approach of centralized systems. Rather than a single, central repository on which clients synchronize, each peer's working copy of the codebase is a bona-fide repository.^[2] Synchronization is conducted by exchanging patches (change-sets) from peer to peer. This results in some striking differences from a centralized system:

• No canonical, reference copy of the codebase exists by default; only working copies.
• Common operations such as commits, viewing history, and reverting changes are fast, because there is no need to communicate with a central server.^[3]
• Each working copy is effectively a remoted backup of the codebase and change history, providing natural security against data loss.^[3]

There are two types of distributed systems: open and closed. Open systems are tuned more to open-source development, and closed systems to traditional, single baseline, development.

Open Systems

An open system of distributed revision control is characterized by its support for independent branches, and its heavy reliance on merge operations. Its general characteristics are:

• Every working copy is effectively a branch.
• Each branch is actually implemented as a working copy, with merges conducted by ordinary patch exchange, from branch to branch.
• Code forking is therefore easier to accomplish, where desired, because every working copy is a potential fork. (By the same token, undesirable forks are easier to mend because, if the dispute can be resolved, re-merging the code is easy.)
• It may be possible to "cherry-pick" single changes, selectively pulling them from peer to peer.
• New peers can freely join, without applying for access to a server.

One of the first open systems was BitKeeper, noteable for its use in the development of the Linux kernel. A later decision by the makers of BitKeeper to restrict its licensing led the Linux developers on a search for a free replacement^[4]. Common open systems now in free use are:

Bazaar Darcs Git Mercurial

Monotone SVK for a full list, see the comparison of revision control software

Closed Systems

A closed system of distributed revision control is based on a Replicated Database. A check-in is equivalent to a distributed commit. Successful commits create a single baseline. An example of a closed distributed system is Code Co-op.

Integration

Some of the more advanced revision control tools offer many other facilities, allowing deeper integration with other tools and software engineering processes. Plugins are often available for IDEs such as IntelliJ IDEA, Eclipse and Visual Studio. NetBeans IDE comes with integrated version control support.

Common vocabulary

Terminology can vary from system to system, but here are some terms in common usage.^[5][6]

Baseline

An approved revision of a document or source file from which subsequent changes can be made. ; Branch : A set of files under version control may be branched or forked at a point in time so that, from that time forward, two copies of those files may be developed at different speeds or in different ways independently of the other. ; Check-out : A check-out (or checkout or co) creates a local working copy from the repository. Either a specific revision is specified, or the latest is obtained. ; Commit : A commit (check-in, ci or, more rarely, install, submit or record) occurs when a copy of the changes made to the working copy is written or merged into the repository. ; Conflict : A conflict occurs when two changes are made by different parties to the same document, and the system is unable to reconcile the changes. A user must resolve the conflict by combining the changes, or by selecting one change in favour of the other. ; Change : A change (or diff, or delta) represents a specific modification to a document under version control. The granularity of the modification considered a change varies between version control systems. ; Change list : On many version control systems with atomic multi-change commits, a changelist, change set, or patch identifies the set of changes made in a single commit. This can also represent a sequential view of the source code, allowing source to be examined as of any particular changelist ID. ; Dynamic stream : A stream (a data structure that implements a configuration of the elements in a particular repository) whose configuration changes over time, with new versions promoted from child workspaces and/or from other dynamic streams. It also inherits versions from its parent stream. ; Export : An export is similar to a check-out except that it creates a clean directory tree without the version control metadata used in a working copy. Often used prior to publishing the contents. ; Head : The most recent commit. ; Import : An import is the action of copying a local directory tree (that is not currently a working copy) into the repository for the first time. ; Mainline : Similar to Trunk, but there can be a Mainline for each branch. ; Merge : A merge or integration brings together two sets of changes to a file or set of files into a unified revision of that file or files.

* This may happen when one user, working on those files, updates their working copy with changes made, and checked into the repository, by other users. Conversely, this same process may happen in the repository when a user tries to check-in their changes.

* It may happen after a set of files has been branched, then a problem that existed before the branching is fixed in one branch and this fix needs merging into the other.

* It may happen after files have been branched, developed independently for a while and then are required to be merged back into a single unified trunk.

; Repository : The repository is where the current and historical file data is stored, often on a server. Sometimes also called a depot (e.g. with SVK, AccuRev and Perforce). ; Reverse integration : The process of merging different team branches into the main trunk of the versioning system. ; Revision : A revision or version is one version in a chain of changes. ; Tag : A tag or release refers to an important snapshot in time, consistent across many files. These files at that point may all be tagged with a user-friendly, meaningful name or revision number. ; Trunk: The unique line of development that is not a branch (sometimes also called Baseline or Mainline) ; Resolve : The act of user intervention to address a conflict between different changes to the same document. ; Update : An update (or sync) merges changes that have been made in the repository (e.g. by other people) into the local working copy. ; Working copy: The working copy is the local copy of files from a repository, at a specific time or revision. All work done to the files in a repository is initially done on a working copy, hence the name. Conceptually, it is a sandbox.

References

See also

• History of software configuration management
• Comparison of revision control software
• Configuration management
• List of revision control software - by category
• Neutral build
• Recombinant text
• Scientific Community Metaphor only adds and records changes, but does not allow retractions of publications
• SCM (Software Configuration Management) - Revision control for software projects
• Software versioning
• Source tracking
• SyncML
• WebDAV
• Versioning file system
• Files-11 filesystem for OpenVMS supports file versioning
• Fossil filesystem for Plan 9 supports intervaled snapshots
• ZFS filesystem for OpenSolaris supports intervaled snapshots

External links

• The Configuration Management Yellow Pages at CM Crossroads
• Revctrl Wiki
• Essays on Version control A collection of essays on version control and software configuration management
• Eric Sink's Source Control HOWTO A primer on the basics of version control
• RevML
• Better SCM Initiative, a web site comparing SCM tools
• Visual Guide to Version Control